The present invention relates to black pigment particles which are non-magnetic, safe and harmless, and excellent in workability and heat resistance.
The black pigment particles according to the present invention are particularly useful as a developing toner, coloring materials for paint, coloring materials for resins, and the like.
Generally black pigment particles are widely used as a developing toner by mixing and dispersing the pigment particles in a resin and then molding, or as coating materials by mixing and dispersing the said pigment particles in a vehicle.
Recently, black pigment particles which are non-magnetic, safe and harmless, and also excellent in workability and heat resistance, has been strongly demanded from the standpoints of enhancement of working efficiency in the age of energy saving, safety and sanitation, and improvement of properties.
For the improvement of workability of the pigment particles, it is imperative that the pigment particles are non-magnetic and have good dispersibility with an appropriate range of size, and are offered in the form of powder which is easy to handle by possessing a high apparent density and an appropriate range of size.
Heat resistance is also requisite for the said black pigment particles because these particles when being used for developing toner are required to be stable in hue even at high temperatures above 150.degree. C., since the developing toner, whose demand is growing with the spread of copying devices in recent years, is heated to a high temperature over 150.degree. C. in its production process.
Hitherto, magnetite particles and carbon black particles have been popularly used as black pigment particles
Thus, the black pigment particles which are non-magnetic, safe and harmless, and also excellent in workability and heat resistance are now most acutely required. The known magnetite particles, because of their magnetism, tend to repeat mutual agglomeration to deteriorate their dispersibility, resulting in poor workability. Also, the magnetite particles begin to transform into maghemite particles at a temperature above 150.degree. C. and are changed in color from black to liver brown, and therefore, they are unsatisfactory in heat resistance. On the other hand, a carbon black, although excellent in heat resistance, is hard to handle and bad in workability because it is a powder comprising ultra-fine particles with a size of 0.01-0.1 .mu.m while having a high apparent density of around 0.1 g/cm.sup.3. It is also pointed out that the carbon black is problematic in the aspect of safety and sanitation such as carcinogenicity thereof, etc.
Accordingly, the technical subject of the present invention is to provide black pigment particles which are non-magnetic, safe and harmless, and also excellent in workability and heat resistance.
As a result of the present inventors' studies for obtaining the black pigment particles described above, it has been found that polycrystalline particles having a mixed composition of Fe.sub.2 TiO.sub.5 and Fe.sub.2 O.sub.3 --FeTiO.sub.3 solid solution, produced by subjecting magnetite particles having their surfaces coated with a titanium compound, a mixture of magnetite particles and a titanium compound, or reduced particles obtained by reducing the hematite particles having their surfaces coated with a titanium compound, to heat-treatment at a temperature of not less than 700.degree. C. under a non-oxidizing atmosphere, and pulverizing the thus treated particles, can fulfill the above-mentioned purpose. The present invention has been attained on the basis of this finding.